1. Field of The Invention
This invention concerns improvements in the mounting of engine components having dissimilar coefficients of thermal expansion, and relates to the mounting of components in high temperature regions of a gas turbine engine.
2. Discussion Of The Prior Art
In the operation of a gas turbine engine, air at atmospheric pressure is initially compressed by a compressor and delivered to a combustion stage. In the combustion stage heat is added to the air leaving the compressor by adding fuel to the air and burning it. The gas flow resulting from combustion of fuel in the combustion stage then expands through a turbine, delivering up some of its energy to drive the turbine and produce mechanical power.
In order to produce a driving torque, the turbine consists of one or more stages, each employing one row of stationary nozzle guide vanes and one row of moving blades mounted on or integral with a disc. The nozzle guide vanes are aerodynamically designed to direct incoming gas from the combustion stage onto the turbine blades and thereby transfer kinetic energy to the blades.
The gases entering the turbine typically have an entry temperature from 850.degree. to at least 1700.degree. C. Since the efficiency and power output of the turbine are related to the entry temperature of the incoming gases there is a trend in gas turbine engine technology to increase the gas temperature. A consequence of this is that the materials of which the blades and vanes are made assume ever-increasing importance with a view to resisting the effects of elevated temperature.
Historically, nozzle guide vanes have been made of metals such as high temperature steels and, latterly, nickel alloys, and it has been found necessary to provide internal cooling passages in order to prevent melting. It has been found that ceramic coatings can enhance the heat resistance of nozzle guide vanes, and, in specialised applications, nozzle guide vanes are being made entirely of ceramic, thus imparting resistance to even higher gas entry temperatures.
However, if the nozzle guide vanes are made of ceramic, which has a different coefficient of thermal expansion to that of a metal supporting structure, then undesirable thermal stresses will be set up between the nozzle guide vanes and their supports when the engine is operating. Such undesirable thermal stresses cannot adequately be contained by cooling.
It is an object of the present invention to provide in the turbine section of a gas turbine engine an assembly of a nozzle guide vane and a supporting structure which is designed to minimise thermal stresses in the assembly when the nozzle guide vane and the supporting structure are made of materials having different coefficients of thermal expansion.